A relay station is considered for most advanced wireless communication systems, such as Institute of Electrical and Electronics Engineers (IEEE) 802.11, IEEE 802.16 and 3GPP LTE. Specifically in IEEE 802.11, task group ah (TGah) recently decided to include a relay feature with it. However, as a relay means multiple transmissions of the same packet, it is desired to minimize any associated overhead. There have been several ideas proposed in IEEE 802.11 TGah for efficient transmission in a relay configuration.
One example is a shared transmission opportunity (TXOP) for the relay to reduce the number of contentions for channel access. FIG. 1 illustrates a downlink relay example. In 102, the access point (AP) sends a downlink DATA frame to the relay. In 104, the relay sends an acknowledgment (ACK). In 106, after receipt of ACK, the AP removes a frame from the buffer, and defers maximum physical protocol data unit (MAX_PPDU)+ACK+2*SIFS before the next event. In 108, in short inter-frame space (SIFS) time, the relay sends DATA with a modulation an coding scheme (MCS) that does not need to be the same (although it may be the same) as the MCS that the AP sent to the relay, depending on the channel condition between the relay and the STA. The relay buffers the frame until successful delivery or reaching of a retry limit.
FIG. 2 illustrates an uplink relay example. In 202, a station (STA) (i.e., a wireless station) sends an uplink DATA frame to the relay. In 204, the relay sends ACK. In 206, after receipt of ACK, the STA removes a frame from the buffer, and defers MAX_PPDU+ACK+2*SIFS before the next event. In 208, in SIFS time, the relay sends DATA with a MCS that may be the same or different from the MCS that the AP sent to the relay. The relay buffers the frame until successful delivery or reaching of a retry limit.
For both the downlink and uplink examples, however, the ACK frame has to be sent from the relay station to the original sender to make sure the data packet is successfully received at the relay station.